The present invention relates to a novel aqueous dispersion of a low-temperature curable cationic electrodeposition resin composition and a process for preparing the same, and more particularly to a novel aqueous dispersion of the low-temperature curable cationic electrodeposition resin composition in which the electrodeposited coating film thereof has an epoxy-acrylic double-layered structure to give a high functionality and the aqueous dispersion is used for the preparation of a cationic electrodeposition paint composition having a low-temperature curability, and a process for preparing the same.
Cationic electrodeposition paint is widely used for primer coating of cars, household electric appliances, and industrial machines. Recent research at home and abroad on cationic electrodeposition paint have concentrated on paints related to energy conservation, safety, and environmental considerations. Also, research on binders of electrodeposition coating have concentrated on developing highly functional and highly efficient electrodeposition paint having various purposes and characteristics. Among the research, studies on reducing organic solvent emissions from paint operations are being developed. The motive for these studies is the regulation of emission of organic solvents for the protection of the environment. As examples of regulations related to the regulation of emission of organic solvents, the regulation for volatile organic compounds (VOC) of the Environment Office in U.S. and the atmospheric purification law in Germany can be noted. The former establishes a guideline for the amount of organic solvents in a paint, while the latter restricts the amount of organic solvents discharged from the paint used for coating cars. Since about 20% of the amount of carbon dioxide discharged in the world are due to the coating industry (for example, organic solvent and combustion by means of a drying oven), the coating industry is a major cause of environmental pollution. Hence, the coating industry has become the subject of supervision.
Presently, as an electrodeposition coating composition for primer coating, epoxy cationic electrodeposition paints which are curable at a temperature above 160.degree. C. are mainly used. However, the body of a vehicle and parts thereof have complicated structures and thus the temperature range for applying the epoxy cationic electrodeposition paints thereto at the above high temperature is wide. Due to these facts, a portion where the coated paints are not sufficiently cured may be generated. On this portion, there may be a problem that the anticorrosive property of the body or parts thereof become poor. Further, when the coated paint is dried at a high temperature, the reduced weight percentage of the coated paint is great and thus the used amount for the coated paint is increased. Accordingly, the costs for the coated paint is increased. In the meantime, these epoxy cationic electrodeposition paints are rich in organic solvents and these solvents are evaporated from the electrodeposition tank, thereby emitting an offensive odor in a workpiece. Also, these solvents are emitted during baking, so environmental pollution problems occur. Further, the conventional epoxy cationic electrodeposition paints, which may be cured at a high temperature, are lacking in properties such as weather-resistance, yellowing resistance, etc.
In the meantime, the present inventors have an invention entitled "IMPROVED CATIONIC ELECTRODEPOSITION COATING COMPOSITION AND A PROCESS FOR PREPARING THE SAME" wherein the electrodeposited coating film of the eletrodeposition coating composition has an epoxy-acrylic double-layered structure, and filed as U.S. patent application Ser. No. 08/928,044 on Sep. 11, 1997 and is now pending in the USPTO. However, the disclosed electrodeposition coating composition in the above U.S. patent application is cured at a temperature above 160.degree. C. and thus can not be cured at a low temperature below 125.degree. C.
Accordingly, the present inventors have made many efforts for obtaining an electrodeposition coating composition which may be cured at a temperature below 125.degree. C. and accomplished the present invention.